Highly fluorinated carboxylic acids are useful emulsifiers in the aqueous emulsion polymerization of fluorinated monomers. In the past perfluorinated low molecular weight carboxylic acids of the general formula CF3—(CF2)n—COO−M+ have been used for this purpose, wherein M+ represents a cation and n represents an integer between 4 and 8. However, alternative fluorinated emulsifiers that are more readily degradable have become of interest for various reasons.
Fluorinated polyether carboxylic acids and partially fluorinated carboxylic acids have been suggested as alternative emulsifiers. In particular highly fluorinated fluoroalkoxy carboxylic acids of the general formula [Rf—O-L-COO—]iXi+ described in (US2007/0015937 to Hintzer et al,), wherein L represents an alkylene group or an aliphatic hydrocarbon group, Rf represents a linear partially or fully fluorinated aliphatic group interrupted with one or more oxygen atoms, Xi+ represents a cation having the valence i and i is 1, 2 or 3, have been found to be useful alternatives of the commonly used perfluorinated alkanoic acids.
Various methods have been described to prepare these highly fluorinated carboxylic acids. For example, (U.S. patent application Ser. No. 2006/0281946 to Morita et al) describes a process based on a ring opening reaction of tetrafluorooxetanes to create acid fluorides which are then converted into carboxylic acids. However, this process is cumbersome and involves various reaction steps. U.S. patent application Ser. No. 2007/0025902 to Hintzer et al describes the preparation of fluorinated carboxylic acids starting from alcohols, using strong oxidizing agents, such as RuO4, OsO4, permanganates or chromium (VI) oxides. While this reaction can be carried out in an industrial scale with good yields, the heavy metal residues originating from the oxidizing agents have to be retained for environmental reasons making this process expensive. Therefore, there is a need for an alternative process for making highly fluorinated carboxylic acids.